The crystal chemistry and FeII-site properties of aluminosilicate garnet solid solutions as revealed by Mossbauer spectroscopy and electronic structure calculations

The three binary garnet solid solutions (Fe3Al2Si3O12)-Al-II-(X3Al2Si3O12)-Al-II (X-II= Mg-II, Mn-II, Ca-II) have been investigated by Fe-57 Mossbauer spectroscopy at 298 and 77 K and by electronic structure calculations in the local spin density approximation. The spectra yield isomer shifts and quadrupole splittings that are typical for Fe-II in the dodecahedral X-site of 222 point symmetry and are similar for each of the three binaries recorded. Conversely, electronic structure calculations based on the experimental crystal structure of the different end-member garnets exhibit pronounced variations in some of the electronic properties of Fe-II that are not reflected in the spectroscopic data. These results are interpreted as indicating that the different X-O bonds in garnet solid solutions retain to a large degree the intrinsic lengths that they possess in their respective end members, and that the Fe-O bond does not change greatly as a function of composition. This is evidence for the state of alternating bonds and not for the virtual crystal approximation in describing the X-O bond types or lengths in aluminosilicate garnet solid solutions. The observed degree and behavior of the Fe-II doublet asymmetry in the Mossbauer spectra for the three solid solution series do not indicate major variations in the anisotropic recoil-free fraction of Fe-II. Variations in doublet asymmetry are more likely a result of complex next-nearest X-site neighbor interactions and/or some degree of short-range cation ordering, though doublets representing different local X-site cation configurations cannot be resolved or fitted to the experimental spectra.